Ataxia -Telangiectasia (A-T) is a rare progressive disease inherited as an autosomal recessive trait. This biochemical genetic disease results in cerebellar ataxia, immunodeficiency and cancer. The defective gene, ATM, encodes a large protein which appears to be confined to the nucleus of the host cells and plays a role in DNA repair which is defective in AT. The goals of this project are to: 1) characterize the cerebellar dysfunction in patients of different ages and with different ATM gene mutations; 2) characterize the T-cell abnormalities in patients; 3) measure the levels of the myo-inositol and the phosphoinositides in cells; and 4) determine whether any of the clinical or laboratory abnormalities can be reversed by supplying for one month more of the phosphoinositide precursor, myo-inositol, to patients as part of their daily diet. This will be accomplished by performing a placebo controlled, double-blind crossover study in which at the beginning and end of each month the cerebellar and T-cell studies will be performed as well as the measurement of phosphoinositides and myo-inositol in cells.